Decision feedback equalizers (DFE) are known in the art for reducing intersymbol interference (ISI) caused in digital data communication channels having narrow bandwidths such as a telephone voice communication channel. A DFE may have a multiple (N-tap) transversal type filter in which digital data is received, decoded and sequentially shifted or delayed through the N-taps at a baud rate (T). The output of the N-taps are multiplied by individual tap coefficients to produce N-products, which are summed to construct an estimated replication of the ISI present in the received signal. The ISI in the received signal is cancelled by subtracting the ISI estimate from it. In order to hone the ISI estimate so as to provide better cancellation, a .DFE may have a transversal filter with adaptive tap coefficients adapting as a function of the error between the true and estimated ISI. A signal which is a faithful reproduction of the desired signal is produced by iteratively adapting tap coefficients until an estimate closely equaling the true ISI is obtained and subtracted from the receive data signal.
U.S. Pat. No. 4,789,994 to Randell et. al. issued Dec. 6, 1988, herein incorporated by reference, teaches an adaptive DFE using a precursor error signal for convergence control. The DFE measures the ISI precursor effect in each baud signal decision output and updates all transversal filter tap coefficients in response to a multiple cursor effect estimate. A sign update algorithm is used for coefficient adaptation but stable convergence of the DFE by selective tap coefficient adaptation is not addressed. Although ISI estimation is improved as the number of filter taps in the DFE is increased, a side effect to this is to also increase the probability of the ISI estimate diverging on start up. The probability of divergence increases due to the added error contributions induced by excessive noise or interference and propagated by the additional filter taps. When the DFE first starts up, all tap coefficients of the filter are set to zero. It is at this time that the process of coefficient modification is of increased concern. If the probability of error in updating the DFE filter coefficients is greater than 0.5, the DFE ISI estimate will not converge. As adaptation proceeds, the coefficients become non-zero and received data errors caused by ISI lead to erroneous coefficient modification and hence more decoding errors leading even further to more decoding errors thus causing the DFE ISI estimation to diverge. In a DFE having a filter with a plurality of taps, such as 30 taps, divergence is inevitable unless steps are taken to prevent it.
U.S. Pat. No. 4,520,489 to Hogge issued May 25, 1985, herein incorporated by reference, provides decision feedback equalization using a variable gain differential amplifier as a level detector and a variable gain adjustment to provide non-delayed positive feedback and bit time delayed intersymbol interference compensating feedback to both enhance the amplitude of the present bit and reduce the ISI of one or more prior bits, but does not address adaptation or convergence of the DFE.
U.S. Pat. No. 4,583,234 to Ramadan issued Apr. 15, 1986, herein incorporated by reference, teaches a DFE having positive feedback to enhance the amplitude of the present data bit as well as providing negative feedback to cancel postcursor ISI. It is silent on convergent DFE coefficient adaptation.
U.S. Pat. No. 4,412,341 to Gersho et. al. issued Oct. 25, 1983, herein incorporated by reference, describes a method and apparatus for avoiding error propagation in ISI cancellation using a phase compensation filter to give receive date estimates and comparing them against uncompensated estimates.
U.S. Pat. No. 4,468,786 to Davis issued Oct. 25, 1983, herein incorporated by reference, teaches an adaptive DFE using a training sequence for stable convergence.